Method of forming low dielectric constant insulation film for semiconductor device

ABSTRACT

A thin film having a low dielectric constant is formed on a semiconductor substrate by plasma reaction using a method including the steps of: (i) introducing a reaction gas into a reaction chamber for plasma CVD processing wherein a semiconductor substrate is placed on a lower stage; and (ii) forming a thin film on the substrate by plasma reaction while reducing or discharging an electric charge from the substrate surface. The discharging can be conducted by forming in the reaction chamber a upper region for plasma excitation and a lower region for film formation on the substrate wherein substantially no electric potential is applied in the lower region to suppress plasma excitation. An intermediate electrode is used to divide the interior of the reaction chamber into the upper region and the lower region. The discharge can also be conducted by lowering the temperature of the lower stage to condense moisture molecules on the substrate surface. Small nanoparticles can dispose on the substrate without interference with an electric charge.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a plasma CVD (Chemical VaporDeposition) method for forming a thin film, and particularly to a methodfor forming a low dielectric constant insulation film for asemiconductor device.

[0003] 2. Description of the Related Art

[0004] A plasma CVD film-forming method is a technique of forming a thinfilm on a substrate in a reaction space by generating plasma by bringingmicrowaves or RF radio-frequency electric power into a reaction chamber.For methods of bringing electric power in, there are the capacitycoupling method, the inductive coupling method, the electromagnetic wavecoupling method and others. FIG. 1 shows an embodiment of plasma CVDequipment of a parallel-flat-plate type using a capacity couplingmethod. By placing two pairs of electrically conductive flat electrodes101, 102 parallel to and opposing each other within a reaction chamber104, applying RF power 105 to one side and grounding the other side, theplasma is excited between these two electrodes to form a film on asubstrate 103. Radio-frequency electric power in a megahertz band of13.56 MHz or 27 MHz or in a kilohertz band of 400 kHz is appliedindependently or by synthesizing them. In addition to this, there arethe ICP method, the ECR method using microwaves, helicon wave plasma,and surface wave plasma, etc. In such film-forming equipment, a methodin which a plasma source is placed at the top and a substrate on which afilm is formed is placed at a lower stage, and the lower stage iselectrically grounded or a bias voltage is applied, is widely used.

[0005] For methods of forming a low dielectric constant film using theplasma CVD, a method of forming a film using Teflon CFx with a lowdielectric constant as a material, a technique which forms a lowdielectric constant film by reducing polarizability by adding fluorineto a Si material and other methods have been reported. In the case oftechniques using fluorine, however, because device reliabilitydeteriorates due to the corrosiveness of fluorine and low adhesion ofthe material, these low dielectric constant films of k<2.5 have not beenput to practical use. When fluorine is not used, to form a lowdielectric constant film of k<2.5, lowering film density by forming afilm porously is required. In conventional plasma CVD methods, however,due to electric phenomena such as a sheath occurring near a wafer causedby the wafer being exposed to plasma and a self-bias occurring at awafer substrate, a film becomes dense and it is difficult to form a lowdielectric constant film of k<2.5 or less.

[0006] Regarding coating methods, examples of forming a low dielectricconstant film of k<2.5 or less have been reported including an exampleof forming a film with a porous structure by controlling sinteringconditions and an example of using a supercritical drying method. Thesemethods have not been put to practical use due to many problems,including film quality.

SUMMARY OF THE INVENTION

[0007] As described above, to integrate semiconductors and to increasethe operating speed, in recent years, low dielectric constant films witha dielectric constant of approximately 2.0 are demanded and forming afilm with a dielectric constant of approximately 2.0 using a coatingmethod has been reported. However, a film formed by the conventionalcoating method has problems that it generally has poor film strength andstability and that film formation costs tend to increase. Generally, athin film formed using a plasma CVD method is of high quality, and thusthe method is used in various fields including manufacturingsemiconductor devices. An object of the present invention is to form alow dielectric constant film with a porous structure comprising Simaterials using a CVD.

[0008] One aspect of the invention is a method for forming a thin filmon a semiconductor substrate by plasma reaction, comprising the stepsof: (i) introducing a reaction gas into a reaction chamber for plasmaCVD processing wherein a semiconductor substrate is placed on a lowerstage; and (ii) forming a thin film on the substrate by plasma reactionwhile reducing or discharging an electric charge from the substratesurface. By reducing or discharging an electric charge from thesubstrate surface, it is possible to prevent nanoparticles generated byplasma reaction from being repelled from the substrate surface, therebydisposing more nanoparticles on the surface.

[0009] In the above, in an embodiment, said reducing or discharging isconducted by forming in the reaction chamber a upper region for plasmaexcitation and a lower region for film formation on the substratewherein substantially no electric potential is applied in the lowerregion to suppress plasma excitation, thereby reducing an electriccharge from the lower region. The above can be achieved when the upperregion and the lower region are divided by an electrically conductiveintermediate plate having plural pores through which the reaction gaspasses, wherein substantially no electric potential is applied betweenthe intermediate plate and the lower stage. Further, when plasmaexcitation is suppressed in the lower region, nanoparticles do notsubstantially increase in size. Thus, small nanoparticles can disposedon the surface without interference with an electric charge, so that afilm having a fine structure with a low dielectric constant can beobtained.

[0010] In another embodiment, said reducing or discharging can beconducted by lowering the temperature of the lower stage to condensemoisture molecules present in the reaction chamber on the substrate,thereby discharging an electric charge from the substrate surface.

[0011] When the intermediate plate and the lower temperature control ofthe lower stage are used in combination, more nanoparticles having asmall diameter can disposed on the surface.

[0012] The present invention can equally be applied to a CVD apparatusfor forming a thin film on a semiconductor substrate by plasma reaction.In an embodiment, a CVD apparatus comprises: (a) a reaction chamber; (b)a reaction gas inlet for introducing a reaction gas into the reactionchamber; (c) a lower stage on which a semiconductor substrate is placedin the reaction chamber; (d) an upper electrode for plasma excitation inthe reaction chamber; and (e) an electrically conductive intermediateplate with plural pores disposed between the upper electrode and thelower stage, said intermediate plate dividing the interior of thereaction chamber into an upper region and a lower region.

[0013] In the above, in an embodiment, the intermediate plate and thelower stage are electrically connected to maintain the intermediateplate and the lower stage at the same voltage. Further, in anotherembodiment, a CDV apparatus further comprises a temperature controllerwhich controls the temperatures of the lower stage, the intermediateplate, and the upper electrode at −10° C.-150° C., 50° C.-200° C., and100° C.-400° C., respectively.

[0014] The present invention can also be adapted to a film formed by theabove-mentioned methods using a gas containing Si (a Si material gassuch as an organosilicon and/or non-organosilicon gas), which film isformed with nanoparticles having a particle size of approximately 50 nmor less (preferably approximately 10 nm or less) and has a lowdielectric constant of approximately 2.5 or lower (preferablyapproximately 2.0 or less). The present invention enables forming of alow dielectric constant film using a plasma CVD method. Use of this lowdielectric constant film as an insulation film for the next-generationhighly integrated semiconductor elements can substantially improve theoperating speed of the semiconductor elements by decreasing delayscaused by capacity between wiring.

[0015] For purposes of summarizing the invention and the advantagesachieved over the prior art, certain objects and advantages of theinvention have been described above. Of course, it is to be understoodthat not necessarily all such objects or advantages may be achieved inaccordance with any particular embodiment of the invention. Thus, forexample, those skilled in the art will recognize that the invention maybe embodied or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

[0016] Further aspects, features and advantages of this invention willbecome apparent from the detailed description of the preferredembodiments which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] These and other features of this invention will now be describedwith reference to the drawings of preferred embodiments which areintended to illustrate and not to limit the invention.

[0018]FIG. 1 is a schematic side view showing a conventional CVDapparatus.

[0019]FIG. 2 is a schematic side view showing an embodiment of theapparatus according to the present invention.

[0020]FIG. 3 is a schematic side view showing an embodiment of theintermediate electrode according to the present invention.

[0021]FIG. 4 is a schematic diagram showing a high intensity X-raydiffuse scattering optical system to measure the size of nano-pores of afilm.

[0022]FIG. 5 is a graph showing the distribution of pore diameters ofExample 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0023] Equipment Configuration

[0024]FIG. 2 shows an embodiment of film-forming equipment used in thisinvention. Two electrically conductive electrodes, an upper electrode 1and an intermediate electrode 2, and three plates of approximately φ250mm at a lower stage 3 on which a wafer substrate 5 of φ200 mm is placedare provided in a vacuum container 4 which is a reaction chamber. Theupper electrode and the intermediate electrode are installed paralleland opposed to each other at an interval of approximately 20 mm, and theintermediate electrode and the lower stage are also installed in thesame way which is parallel and opposed to each other at an interval ofapproximately 20 mm.

[0025] On the upper electrode, the intermediate electrode and the lowerstage, an independent temperature regulating mechanism is installedrespectively and stated temperatures are maintained. Temperatures aremaintained at approximately 100° C.-400° C. for the upper electrode,approximately 50° C.-200° C. for the intermediate electrode andapproximately 10° C.-150° C. for the lower stage.

[0026] A material gas (source gas) containing silicon (such as anorganic silicon gas expressed by at least SiαOβCxHy wherein α is aninteger of >0 and β, x and y are integers of>=0) and added gases such asN20, He and Ar are controlled at a stated flow through feeding devices6˜8 and flow regulators 9˜11, and after these gases are mixed, they arebrought in an inlet 12 at the top of the upper electrode as a reactiongas. On the upper electrode, 500˜10,000 pores of φ0.5 mm (3,000 poresformed in the mode for carrying out this invention) are formed, and thereaction gas brought in flows through these pores into a reaction space.The reaction space is exhausted by a vacuum pump and a pre-determinedfixed pressure is maintained within the limits of 100 Pa-5000 Pa withinwhich a plasma state can be maintained.

[0027] Pulse-modulated radio-frequency electric power of 13.56 MHz isapplied to the upper electrode, and the intermediate electrode and thelower stage are electrically grounded. Between the upper electrode andthe intermediate electrode, the plasma is excited using the capacitycoupling method.

[0028] As the intermediate electrode 2, an electrode shown in FIG. 3which is an electrically conductive plate of 5 mm in thickness with500-10,000 (preferably 1,000-5,000) relatively large pores 20 formed(3,000 pores formed in the mode for carrying out this invention) isused. This intermediate electrode is designed so that it separates theupper region which is in a plasma state with a relatively hightemperature, from the lower region beneath the intermediate electrodeand at the same time a reaction gas flows from the upper region to thelower region.

[0029] In an embodiment, the intermediate electrode may have thefollowing profiles:

[0030] (a) The material of the intermediate electrode: conductivematerial such as Al and AlN.

[0031] (b) The size of pores: Approximately 0.5-20 mm.

[0032] (c) The size of the intermediate electrode: Approximately thesize of a showerhead.

[0033] (d) The porosity of the intermediate electrode: Approximately5-100% (preferably 20-100%) (as long as the material functions as anelectrode, the configuration can be in the form of a net.)

[0034] (e) The position of the intermediate electrode: The distancebetween the upper electrode and the intermediate electrode:Approximately 5-30 mm; the distance between the intermediate electrodeand the lower electrode: Approximately 10-100 mm.

[0035] (f) The thickness of the intermediate electrode: Approximately2-30 mm.

[0036] (g) Production of the intermediate electrode: Production methodincludes, but is not limited to, mechanically processing a material suchas an Al-based material, and then subjecting the surface of theprocessed material to anodization treatment.

[0037] Two Regions Divided by Intermediate Electrode

[0038] In the upper region, plasma is excited between the upperelectrode and the intermediate electrode and a material gas causes apolymerization reaction. Since nanoparticles are generated in the upperregion, their size is determined by a reaction occurring in the upperregion. A reaction gas is brought into a reaction space (the upperregion), and until it reaches the intermediate electrode, nanoparticlesincrease in size. The size of nanoparticles is preferably as small asseveral nm in diameter, and a required nanoparticle diameter can beobtained by controlling time required for a reaction gas to pass throughthe upper region and reactivity of the upper region. The time forpassing through the upper region, i.e., the time for staying in thisregion, can be controlled by a total flow of a reaction gas per unit oftime. The reactivity of the upper region can be changed mainly byregulating RF power. Alternatively, using pulse-modulated RF power withan ‘on time’ of approximately 50 msec, particle diameter can becontrolled by a method of growing nanoparticles by turning the RF poweron. As a method of reducing a particle diameter, a minute particle ofφ10 nm or less can be generated by reducing the ratio of a Si materialgas to be incorporated into a reaction gas.

[0039] In the above, in an embodiment, the flow rate of reaction gas inthe upper region may be approximately 10-1000 sccm. The RF power exertedin the upper region may be approximately 10 W-1500 W. Whenpulse-modulated power is applied in the upper region, pulse intervalsmay be 10-200 msec for activation and 20-100 msec for deactivation. Inan embodiment, the size of nanoparticles passing through theintermediate electrode may be approximately 0.5-50 nm.

[0040] Nanoparticles generated in the upper space pass through a numberof pores formed in the intermediate electrode along with a reaction gasand enter the lower region. Since the electric potential of theintermediate electrode and the lower stage is substantially the same orif they are different, the difference in electric potential is smallenough not to excite plasma, the RF power by which plasma is exciteddoes not exist. Ionized particles which enter the lower region become innon-ionized form state after moving in a distance corresponding to themean free path and colliding with neutral gas molecules. A distancewhich an ionized particle can move freely, i.e., the mean free path, canbe obtained by the following formula with P (Pa) as a pressure inside areaction chamber:

λ(mm)=44/p

[0041] Although, in a high vacuum region of approximately 10⁻⁴ this freepath is hundreds of meters which is very long, by increasing the processpressure to 100 Pa-1000 Pa, however, the mean free path shortens to avery short 0.44 mm-0.044 mm. Consequently, plasma ions having passedthrough the intermediate electrode collide with neutral molecules in ashort period of time, preventing the plasma state from spreading intothe lower region.

[0042] The intermediate electrode divides the reaction chamber into twosections. That is, the lower region of the reaction chamber has nosignificant presence of plasma, whereas the upper region of the reactionchamber has plasma. If plasma exists significantly in the lower region,all portions exposed to plasma, including a wafer substrate,nanoparticles, and the lower electrode, become negatively charged.Accordingly, nanoparticles are repelled from the surface of the wafersubstrate by the force of a static charge, and thus nanoparticles cannotdispose on the surface. By using the intermediate electrode, theintensity of the static charge can be suppressed in the lower region,and disposition of nanoparticles can be enhanced. Further, due to nosignificant presence of plasma in the lower region, nanoparticles do notsignificantly increase in size and no new nanoparticles are generated inthe region. By disposing small nanoparticles on the surface of the wafersubstrate, a film having a fine structure can be formed. If largenanoparticles are disposed on the surface, a film has a coarse structureand when being processed for wiring, a processed surface becomes rough,lowering the quality of a device.

[0043] The use of an intermediate electrode can effectively eliminatethe above-mentioned static charge phenomena and can suppress the growthof nanoparticles in the lower region. However, even if no plasma isexcited in the lower region, static charge phenomena cannot completelybe eliminated, resulting in formation of a soft film. The lower stagetemperature control described below makes it possible to fully releasethe electric charge of nanoparticles to the lower stage. When the lowerstage temperature control is not conducted, by applying bias voltage tothe susceptor, it is possible to enhance the strength of a film.

[0044] Lower Stage Temperature Control

[0045] Static charge phenomena can be eliminated by lowering thetemperature of the lower stage independently of or in combination withthe use of the intermediate electrode. The mechanism can be explained asfollows:

[0046] When the intermediate electrode, if used, and the lower stage arekept at relatively low temperatures, moisture condenses in the lowerregion and adheres to particles formed in the upper region. Moisture ispresent in the reaction chamber as a result of the reaction betweenoxygen and hydrogen. That is, when a material gas containing hydrogen asa constituent element in its molecule (such as organosilicon orhydrocarbon) is used or hydrogen is added to an additive gas, thehydrogen and oxygen react to produce H2O in the reaction gas. Themoisture condenses when being cooled in the lower region. The vaporpressure of water at 12° C. is 10 Torr (133 Pa). Thus, if the pressureof the reaction chamber is approximately 10 Torr, moisture present inthe reaction gas condenses when the temperature is lower than 12° C. Thecondensed moisture adheres to nanoparticles generated in the upperregion and the surface of the water substrate. The amount of moisturecondensed can be controlled by lowering the temperature of the lowersusceptor, increasing the reaction pressure, decreasing the flow of gas,and/or adding hydrogen to generate more moisture, etc.

[0047] When nanoparticles accumulate on a wafer, minute particles thesize of several tens of nanometers or less do not adhere to a wafersubstrate if affected by static charge. By lowering temperatures of thelower space and the lower stage, moisture and material gas by-productmolecules (e.g., alcohol such as CH3OH and C2H5OH, ether, andorganosilicon, SiOxHy), etc. are caused to adhere to a wafer, and theelectric charge charged on a wafer is eliminated, whereby morenanoparticles are accumulated on a wafer.

[0048] In an embodiment, an intermediate electrode is used and the lowerstage is cooled. This combination is effective to dispose smallnanoparticles on a wafer substrate. By maintaining the lower stage andthe intermediate electrode at the same electric potential and by keepingthe reaction chamber in high pressure, dividing a reaction space by theintermediate electrode becomes effective. Spreading into the lowerregion plasma excited in the upper region can be prevented and the upperspace and the lower space can be maintained at different temperatures,i.e., the upper space at a high temperature and the lower space at a lowtemperature. In this method, it becomes possible to form nanoparticlesby facilitating a vapor-phase polymerization reaction by increasingreactivity in the upper space, and in the lower space to end thereaction and at the same time to condense moisture contained in areaction gas.

[0049] Facilitation of moisture condensation can be realized without theuse of an intermediate electrode. If no intermediate electrode is used,the lower stage may be cooled to a temperature lower than roomtemperature while the upper electrode is heated to approximately 150° C.or higher. In order to maintain a lower region in the reaction chamberat a low temperature, the distance between the upper electrode and thelower stage needs to be more than approximately 40 mm.

[0050] Subsequent Treatment

[0051] The thin film formation process takes 1-20 minutes. After a filmis formed, a wafer is transported to another vacuum container and isthermally treated. A film-formed wafer is transported to a vacuumcontainer in a nitrogen atmosphere and is thermally treated for 10seconds to 5 minutes at a temperature of approximately 200-450° C. atreduced pressure (approximately 10-500 Pa). During this treatment, HMDS(Hexamethyldisilano: Si₂(CH₃)₅) is brought into this container, andhydrophobicity treatment is performed to suppress the hygroscopicity ofa film.

[0052] Effects of Invention

[0053] Using the above-mentioned method, it becomes possible in anembodiment to cause a plasma reaction in a reaction gas in a state wherea wafer substrate is not exposed to plasma, and furthermore, in the sameembodiment or in another embodiment, to accumulate a product on a wafersubstrate in a space where there is substantially no plasma.Consequently, no significant electric phenomena (such as a sheathoccurring near a wafer caused by a wafer being exposed to plasma and aself-bias at a wafer substrate) can occur, and it becomes possible toeliminate plasma damage. In the present invention, a film having adielectric constant as low as 1.2-2.5 can be produced.

[0054] This invention enables forming of a low dielectric constant filmusing a plasma CVD method. Use of this low dielectric constant film asan insulation film for the next-generation highly integratedsemiconductor elements can substantially improve the working speed ofthe semiconductor elements by decreasing delays caused by capacitybetween wiring.

[0055] Analysis of Film

[0056] In the present invention, nanoparticles deposit effectively on asubstrate, thereby forming a porous film having nano-pores (with adiameter of 50 nm or less, preferably 10 nm or less, or with a mediandiameter of 10 nm, preferably 1 nm). The size of the nano-pores issimilar to that of the nanoparticles. The size of the nano-pores can bemeasured by a high intensity X-ray diffuse scattering optical system(e.g., ATX-E™, Rigaku Denki X-Ray Laboratory in Japan). FIG. 4 shows aschematic diagram of this system. The angle of divergence and theintensity of incoming X-ray are 0.045 and 10⁹ cps, for example. X-raydiffuse scattering data are evaluated by comparing measured data withthe theoretical intensity of scattering. Practically, the averagediameter and the distributions of scattering objects can be determinedby calculating the intensity of X-ray diffuse scattering based on ascattering function in for spherical scattering objects having variousdiameters.

[0057] Comparison with Other Techniques

[0058] In the case of ion implantation equipment, there is an examplewhich uses an intermediate electrode as a grid electrode in a spacebetween a plasma source and a wafer. The object of the ion implantationequipment is to irradiate ions such as ionized phosphorus, boron, etc.on a Si substrate to implant (dope) these ions on a substrate surface.Consequently, to enable ions to reach a wafer placed on a stage withoutcolliding with other molecules, it is designed that the pressure in areaction chamber is maintained at a high vacuum of 10⁴ Pa or less, themean free path of molecules is long to make the number of molecularcollisions sufficiently small. Additionally, by applying a voltagedifference between an intermediate electrode and a wafer stage, ionspassing through the intermediate electrode are selected.

[0059] In the ion implantation equipment, since the collision of atomsand molecules in a reaction space is suppressed, a polymerizationreaction in a vapor phase does not occur. Additionally, because ions attheir high-energy state reach a wafer, these ions are not accumulated onthe wafer and are doped inside the wafer. In this invention, byincreasing the atmospheric pressure in a reaction space to facilitate apolymerization reaction in a vapor phase, minute particles are generatedin a vapor phase. Additionally, these minute particles deposit on awafer without penetrating inside a wafer substrate.

[0060] Experimental Results

EXAMPLE 1

[0061] 20 sccm of TEOS as a material gas and 80 sccm of O2 as an addedgas, Ar:50 sccm, He:50 sccm were mixed, and the mixed gas was broughtinto a reaction chamber as a reaction gas. The pressure within thereaction chamber was maintained at 2×10 ³ Pa by constantly exhaustingthe gas with a vacuum pump. 300W 13.56 MHz RF power was applied to anupper electrode. A temperature of the upper electrode, an intermediateelectrode and a lower stage was adjusted at a fixed temperature of 170°C., 50° C. and 0° C. respectively. A wafer substrate set in the lowerstage was inserted into a vacuum container for thermal treatment after afilm was formed, and drying in a N₂ atmosphere and hydrophobicitytreatment by HMDS were performed. The measured dielectric constant of afilm formed under these conditions was 2.05.

EXAMPLE 2

[0062] 20 sccm of Dimethyldimetoxysilane (DM-DMOS):(CH₃)₂Si(OCH₃)₂ as amaterial gas, 100 sccm of O2 as an added gas, Ar:50 sccm and He:50 sccmwere mixed, and the mixed gas was brought into a reaction chamber as areaction gas. The pressure within the reaction chamber was maintained at2×10³ Pa by constantly exhausting the gas by a vacuum pump. 800W 13.56MHz RF power was applied to an upper electrode. A temperature of theupper electrode, an intermediate electrode and a lower stage wasadjusted at a fixed temperature of 170° C., 50° C. and 0° C.,respectively. A wafer substrate set in the lower stage was inserted in avacuum container for thermal treatment after a film was formed, anddrying in a N₂ atmosphere and hydrophobicity treatment by HMDS wereperformed. The measured dielectric constant of a film formed under theseconditions was 1.90, and the film had nano-pores having a diameter ofapproximately 10 nm or less and the median diameter was approximately 1nm, which represented the size of nanoparticles (FIG. 5, obtained by ahigh intensity X-ray diffuse scattering optical system, ATX-E™, RigakuDenki X-Ray Laboratory in Japan).

EXAMPLE 3

[0063] 20 sccm of Dimethyldimetoxysilane (DM-DMOS):(CH₃)₂Si(OCH₃)₂ as amaterial gas, 100 sccm of O2 as an added gas, Ar:50 sccm and He:50 sccmwere mixed, and the mixed gas was brought into a reaction chamber as areaction gas. The pressure within the reaction chamber was maintained at2×10³ Pa by constantly exhausting the gas by a vacuum pump.Pulse-modulated 400W 13.56 MHz RF power was applied to an upperelectrode. A temperature of the upper electrode, an intermediateelectrode and a lower stage was adjusted at a fixed temperature of 170°C., 50° C. and 0° C., respectively. A wafer substrate set in the lowerstage was inserted in a vacuum container for thermal treatment after afilm was formed, and drying in a N₂ atmosphere and hydrophobicitytreatment by HMDS were performed. The measured dielectric constant of afilm formed under these conditions was 1.90.

[0064] [Comparison]

[0065] An experiment of forming a low dielectric constant film using aplasma CVD method of a parallel-flat-plate type was conducted. FIG. 1shows an embodiment of plasma CVD equipment used for a film-formingexperiment. With an electrically conductive circular plate of φ250 mmused as a lower stage and electrically conductive circular plate of φ250mm having a limitless number of pores used as an upper electrode, theseplates were installed parallel to and opposing each other at an intervalof 24 mm within a reaction chamber. A temperature of the lower stage waskept at 400° C. at all times. The gas within the reaction chamber wasconstantly exhausted using a vacuum pump and the pressure was maintainedat a stated pressure.

[0066] The wafer of a substrate on which a film is formed was set in thelower stage, 120 sccm of Dimethyldimetoxysilane(DM-DMOS):(CH₃)₂Si(OCH₃)₂ and He: 100 sccm were mixed, and the mixed gaswas brought into a reaction chamber as a reaction gas through the poresof the upper electrode. A pressure within the reaction chamber wasmaintained at 6.7×10² Pa, the lower stage was electrically grounded and1200W 13.56 MHz RF power was applied to an upper electrode to form afilm on a wafer substrate. The measured dielectric constant of a filmformed under these conditions was 2.76.

[0067] It will be understood by those of skill in the art that numerousand various modifications can be made without departing from the spiritof the present invention. Therefore, it should be clearly understoodthat the forms of the present invention are illustrative only and arenot intended to limit the scope of the present invention.

What is claimed is:
 1. A method for forming a thin film on asemiconductor substrate by plasma reaction, comprising the steps of:introducing a reaction gas into a reaction chamber for plasma CVDprocessing wherein a semiconductor substrate is placed on a lower stage;and forming a thin film on the substrate by plasma reaction whilereducing or discharging an electric charge from the substrate surface.2. The method according to claim 1, wherein said reducing or dischargingis conducted by forming in the reaction chamber a upper region forplasma excitation and a lower region for film formation on the substratewherein substantially no electric potential is applied in the lowerregion to suppress plasma excitation, thereby reducing an electriccharge from the lower region.
 3. The method according to claim 2,wherein the pressure of the reaction chamber is 100-5000 Pa.
 4. Themethod according to claim 2, wherein the upper region and the lowerregion are divided by an electrically conductive intermediate platehaving plural pores through which the reaction gas passes, whereinsubstantially no electric potential is applied between the intermediateplate and the lower stage.
 5. The method according to claim 4, whereinthe intermediate plate and the lower stage are electrically connected tomaintain the intermediate plate and the lower stage at the same voltage.6. The method according to claim 3, wherein the reaction gas comprises asource gas which is introduced into the upper region of the reactionchamber.
 7. The method according to claim 4, wherein an upper electrodeis placed above the intermediate plate in the upper region, and plasmais excited between the upper electrode and the intermediate plate. 8.The method according to claim 7, wherein pulse-modified power is appliedbetween the upper electrode and the intermediate plate.
 9. The methodaccording to claim 1, wherein the reaction gas comprises Si.
 10. Themethod according to claim 1, wherein said reducing or discharging isconducted by lowering the temperature of the lower stage to condensemoisture molecules present in the reaction chamber on the substrate,thereby discharging an electric charge from the substrate surface. 11.The method according to claim 10, wherein plasma is excited between anupper electrode and the lower stage, the temperature of the lower stagebeing lower than room temperature, the temperature of the upperelectrode being 150° C. or higher.
 12. The method according to claim 10,wherein the moisture molecules are produced from oxygen and hydrogenpresent in the reaction gas.
 13. The method according to claim 3,wherein said reducing or discharging is further conducted by loweringthe temperature of the lower stage to condense moisture moleculespresent in the reaction chamber on the substrate, thereby discharging anelectric charge from the substrate surface.
 14. The method according toclaim 13, wherein plasma is excited between an upper electrode and thelower stage, the temperature of the lower stage being in the range of−10° C.-150° C., the temperature of the intermediate plate being in therange of 50° C.-200° C., the temperature of the upper electrode being100° C. or higher, wherein the temperature of the lower stage is lowerthan that of the intermediate plate and the upper electrode.
 15. A CVDapparatus for forming a thin film on a semiconductor substrate by plasmareaction, comprising: a reaction chamber; a reaction gas inlet forintroducing a reaction gas into the reaction chamber; a lower stage onwhich a semiconductor substrate is placed in the reaction chamber; anupper electrode for plasma excitation in the reaction chamber; and anelectrically conductive intermediate plate with plural pores disposedbetween the upper electrode and the lower stage, said intermediate platedividing the interior of the reaction chamber into an upper region and alower region.
 16. The apparatus according to claim 15, wherein theintermediate plate and the lower stage are electrically connected tomaintain the intermediate plate and the lower stage at the same voltage.17. The apparatus according to claim 15, further comprising atemperature controller which controls the temperatures of the lowerstage, the intermediate plate, and the upper electrode at −10° C.-150°C., 50° C.-200° C., and 100° C. or higher, respectively.
 18. A filmformed by a method of claim 1 using a silicon-containing gas, which hasnano-pores having a median diameter of 10 nm or less and has a lowdielectric constant of 2.5 or lower.